1. Field of the Invention
The present invention relates to an upgrading method of a law-rank coal such as brown coal, lignite or sub-bituminous coal.
2. Description of the Prior Art
Coals are used generally as fuels or starting materials in the chemical industry. The major part of them is high-rank coals such as bituminous coal. About 1/4 of the total coals produced all over the world is low-rank coals such as brown coal. However, they are not used practically satisfactorily, since some of them have a water content of as high as 70%.
As processes for reducing the water content of coal, there has been known a process for heat-treating coal disclosed in the specification of Japanese Patent Publication No. 11596/1982 of Mitsubishi Heavy Industry Co., Ltd. laid open on Dec. 16, 1980. This process is characterized in that coal is heated to a final temperature of 300.degree. to 500.degree. C. rapidly at a temperature rise rate of at least 100.degree. C./min and then it is cooled rapidly to 250.degree. C. or lower at a temperature lowering rate of at least 50.degree. C./min. In this known process, it is difficult to dehydrate a large amount of coal at once owing to the specific temperature control of coal required.
Some of the low-rank coals have an ash content of as high as more than 20%. For the removal of ash from low-rank coals, there may be mentioned a process disclosed in the specification of Shell Oil's U.S. Pat. No. 4,153,419 "Agglomeration of coal fines" granted on May 8, 1979. However, this patent teaches merely a technique of combining a previously prepared tar with coal fines and agglomerating only the coal fines in an aqueous slurry. The object of this patent is not the dehydration of coal.